1. Field of the Invention
This invention relates broadly to telecommunication systems. More particularly, this invention relates to high speed modems incorporating secondary channels as well as to trellis encoders in the transmitters of such modems and Viterbi decoders in the receivers of such modems.
2. State of the Art
In transmitting data over a channel, it is desirable to code data in manners which will provide as much gain as possible. Various different coding schemes are known in the art. One such coding scheme well known in the art is "trellis encoding". Broadly speaking, trellis encoders are "state machines" which are embodied in the software or firmware of a microprocessor or digital signal processor; i.e., any of a plurality of inputs are given as software inputs to the trellis encoder, and in response thereto and to the state in which the trellis encoder is presently situated, the trellis encoder provides a particular output and updates its state. Typically, the manner in which the trellis encoder decides the appropriate output and the state into which it is updated is by reference to a look-up table (often embodied in firmware) which has the input and present state as inputs and the output and new state as outputs. Details of trellis encoders may be seen with reference to U.S. Pat. No. 4,713,817 to Wei, and with reference to A. R. Calderbank and N. J. A. Sloane, "New Trellis Codes Based on Lattices and Cosets": IEEE Trans. Inform. Theory; Vol. IT-33, pp. 177-195 (1987).
Recently, in U.S. Pat. No. 4,891,806 to Farias et al., it was proposed that a modem include a secondary channel for diagnostic and/or other purposes. The prior art modem system as generally proposed is set forth in the prior art FIG. 1. There it is seen that the primary channel data 12 are inserted into a trellis encoder 13 and a mapper 14 of modem 15. The encoded data 16 are then multiplexed by a multiplexer or switch 18 with data from the secondary channel 20 which was also mapped by mapper 14. The multiplexed data are then sent over a line 25. On the receiving side, the multiplexed data stream is demultiplexed by a demultiplexer or switch 28, and the primary channel data 30 are fed into a Viterbi decoder 32 of modem 35 for decoding, while the secondary channel data stream 37 is decoded by a quadrant decoder 39 separately. According to the prior art disclosure, the secondary data are transmitted every forty-ninth frame, and the primary data channel output of the trellis encoder/mapper 14 is stopped every forty-ninth frame to permit the insertion into the data stream of the secondary data channel output of mapper 14. Such an arrangement not only undesirably requires the stopping and starting of the trellis encoder, but also does not admit of the "distribution preserving Tomlinson" (DPT) encoding and decoding advances set forth in the aforementioned parent applications hereto. In particular, assuming the simplest possible non-trivial channel polynomial h(D)=1+h.sub.1 (D) with no other impairments, the channel's response to the transmitted sequence {x.sub.k } at discrete time k is x.sub.k +h.sub.1 x.sub.k-1. Now, assuming that the secondary channel data is multiplexed into the data stream after the DPT encoding of the primary data; i.e., instead of transmitting {x.sub.k }, another sequence {w.sub.m } is interleaved (e.g., every third baud) with {x.sub.k }, then the transmitted sequence would be: EQU . . . . 0, 0, 0, x.sub.0, x.sub.1, w.sub.0, x.sub.2, x.sub.3, w.sub.1, x.sub.4, . . .
Then, the channel's response to w.sub.0 is w.sub.0 +h.sub.1 x.sub.1, while the channel's response to x.sub.2 is x.sub.2 +h.sub.1 w.sub.0 ; neither response being what would be desired or anticipated by the precoder.